Conventionally, an image forming apparatus is known that, by switching resolution, can form a high resolution image (for example, of 1200 dpi) and a low resolution image (for example, of 600 dpi).
In a case in which the rotation speed of a polygon mirror is the same when forming an image of 600 dpi and when forming an image of 1200 dpi, the rotation speed of a photosensitive drum at 1200 dpi is set at ½ that at 600 dpi. In addition, when the integrated light intensity of the laser scanning the photosensitive drum is the same in both 600 dpi and 1200 dpi directions, the laser light intensity at 1200 dpi is set at half that at 600 dpi, and furthermore, it is necessary to double the frequency of the image clock that drives the laser. By these adjustments, a 1200 dpi image is formed in both the main scanning direction and the sub-scanning direction.
However, when forming an image, it is usually necessary to form the image starting from the same position, and therefore horizontal synch and vertical synch must be obtained. If the horizontal synch is not gotten correctly, the image shifts in the horizontal direction.
In order to obtain the horizontal sync, there is a method whereby, when scanning the photosensitive drum with a light beam from the polygon mirror, the horizontal sync signal is output when the light beam is detected by photo detectors provided near the edges of the photosensitive drum.
However, if the intensity of the light beam is switched according to the resolution, the horizontal sync signal shifts because the configuration is such that the horizontal sync signal is output when the integrated light intensity of the light beam directed onto the photo detectors reaches a certain value. In other words, when the light intensity is high, the horizontal sync signal is output relatively quickly, and when the light intensity is low, the horizontal sync signal is output relatively slowly.
In order to solve this problem, a technology is proposed (Japanese Laid-Open Patent Publication No. 8-304723) that adjusts the light intensity, varying the intensity of the light beam according to the resolution when scanning the photosensitive drum while maintaining the intensity of the light beam at a constant level when the light beam scans the photo detectors provided at the edges of the photosensitive drum. In other words, this technique requires switching the light intensity not only when changing the resolution but also during a single scan.
However, in a color image forming apparatus, equipped with a plurality of lasers, the lasers are allocated to each of the colors, e.g., cyan, magenta, yellow, and black. Moreover, in order to reduce the number of polygon mirrors, in some cases the plurality of lasers are scanned in different directions (e.g., Japanese Laid-Open Patent Publication No. 2004-058492). For example, while the yellow laser scans toward the right, the magenta laser scans to the left. In an instance such as this, any shift in the position at which an image is drawn due to differences in resolution described above (that is, differences in light intensity) is doubled because each shift occurs in the opposite direction. Moreover, this shift is expressed as color shift, and thus seriously degrades the quality of the formed image.
There is a possibility that the foregoing problem can be solved by application of the method described in Japanese Laid-Open Patent Publication No. 8-304723. However, since this method requires two circuits, i.e., one circuit that sets the light intensity when scanning the photosensitive drum and another circuit that sets the light intensity when scanning the photo detectors, circuit scale increases, leading unavoidably to an increase in cost.